Polyurethane foams are made via an exothermic reaction of polyols, water and isocyanates. The temperature of the core of the foam could reach as high as 350.degree. F. As a result a thermo/oxidative degradation of the center of the foam occurs resulting in discoloration ("scorching") of the center of the foam. This scorched part of the foam is an economic loss to the producers since it cannot be used and is often discarded.
To minimize this loss the industry resorts to the addition of anti-oxidants such as butylated hydroxyl toluene (BHT) and tetra bis{methylene(3,5 ditertiary butyl-4-hydroxyl cinnamate}methane. Some use anti-oxidants with trialkyl phosphite but these additives provide for marginal improvement in scorch.
In addition to their marginal effectiveness, some of these additives such as BHT, the most widely used, undergo sublimation and oxidation on exposure to the atmosphere and cause yellowing of the finished composite articles (such as rugs when the foam is used as an underlay). This yellowing is undesirable not only for reasons of appearance but also because it imposes additional costs to treat the rug or other article to remove the yellowing. As a consequence, the elimination of such antioxidants is mandated by many industries.
In Japanese Kokai: 76 14,950 (C.A. 25:7057n) are described elastomers containing hydrazides as anti-oxidants. Japanese 59 15,446 (C.A. 100:211546d) describes the use of hydrazides and semicarbazides as stabilizers for polyurethane filaments. Japanese Kokai: 77 15,549 (C.A. 88:106522d) describes discoloration resistant urethane rubber by the addition of hydrazides. All of the above references deal with elastomeric or fibrous urethanes and not with polyurethane foams. Polyurethane foam technology and composition are significantly different from those of elastomers and fibers. Also, none of the references describe polyesters or polyethers containing hydrazide segments in its structure as a key compound in making oxidation stabilized polyurethane. None of the references disclose or make a reference to the role of catalyst or whipped air in the oxidation of polyurethanes; neither do they mention the role of phosphites as synergists to the hydrazides in stabilizing polyurethanes.
There is thus a need for technology which enables the production of polyurethane foam while reducing the incidence of scorching.
It is also advantageous to be able to make polyurethane elastomers and fibers from precursor materials that are inherently resistant to oxidation and discoloration, thereby eliminating the need for additives that are in many cases fugitive and are depleted over time.